Classical cannabinoids such as the marijuana-derived compound Δ9-tetra-hydrocannabinol, (Δ9-THC) exert their pharmacological effects through interaction with specific members of the G-protein coupled receptor (GPCR) family. To date, two cannabinoid receptors have been cloned and characterized: CB1, a receptor found in the mammalian brain and to a lesser extent in peripheral tissues; and CB2, a receptor found primarily in the peripheral tissues, particularly in cells of the immune system. Several endogenous ligands for these cannabinoid receptors, known as endocannabinoids, have been identified. For a review see Hanus, L. O., Discovery and isolation of anandamide and other endocannabinoids, Chem. Biodivers. (2007) 8:1828-41.
Compounds that are modulators of one or both of the cannabinoid receptors have been shown to produce a variety of pharmacological effects that may be of therapeutic benefit in humans (see, for example, Mackie, K., Cannabinoid receptors as therapeutic targets, Ann. Rev. Pharmacol. Toxicol. (2006) 46: 101-122; Pertwee, R. G., The therapeutic potential of drugs that target cannabinoid receptors or modulate the tissue levels or actions of endocannabinoids, AAPS J. (2005) 7:E625-654). The cannabinoid receptor modulator can be an agonist, an inverse agonist or a neutral antagonist, and may interact at the same (orthosteric) site as the endogenous ligand, or at a different (allosteric) site.
Activation of the CB1 receptor in the brain is believed to mediate undesirable psychotropic effects associated with Δ9-THC and other centrally acting cannabinoid ligands. As a result, there has been considerable interest in developing compounds that possess high affinity and selectivity for the CB2 receptor (see for example, Raitio, K. H. et al., Targeting the Cannabinoid CB2 Receptor: Mutations, Modelling and Development of selective CB2 ligands, Curr. Med. Chem. (2005) 12: 1217-37). CB2 receptor agonists have shown efficacy in preclinical models of neuropathic and inflammatory pain and may also find application in cancer, multiple sclerosis, osteoporosis, Alzheimer's disease, liver disease and diabetes (Mackie, K.; Ross R A; CB2 cannabinoid receptors: new vistas, Br. J. Pharmacol. (2008) 153: 177-78 and references cited therein). There is an ongoing need to identify new CB2 ligands that exhibit greater receptor selectivity, improved drug-like properties and, for some indications, restriction to the periphery with low or minimal effects on the central nervous system (CNS).